1. Field of the Invention
The present invention relates to a driving voltage output circuit for a display panel such as an active matrix liquid crystal panel.
2. Description of the Related Art
Alternate current (AC) driving is known to be a necessary condition for maintenance of the long-term reliability of liquid crystal panels. U.S. Pat. No. 5,973,660 to Hashimoto (Japanese Patent Application Publication No. 10-062744, now Japanese Patent No. 3056085) describes a drive circuit with high-voltage circuits for generating and selecting driving voltages above a common reference voltage and separate low-voltage circuits for generating and selecting driving voltages below the common reference potential. The drive circuit also includes switches that can route the image data for each column of pixels through either the high-voltage or the low-voltage circuits. The switches enable odd-numbered columns to be driven high while even-numbered columns are driven low, or even-numbered columns to be driven high while odd-numbered columns are driven low. Various AC driving schemes are employed: in frame inversion, the switches change once per frame; in one-line dot inversion, the switches change once per line; in two-line dot inversion, the switches change once every two lines.
This driving method conserves power and enables both the high and low driving voltages to be pre-tailored to the gamma curve of the liquid crystal.
The high- and low-voltage circuits also include operational amplifiers operating as voltage followers that output the selected driving voltages on the selected signal lines. The operational amplifiers shown in U.S. Pat. No. 5,973,660 (FIGS. 10 and 11) have a simple circuit configuration consisting of a differential input stage and a single-ended output stage. The output stage includes a field-effect transistor with its source connected to the power supply or ground, its gate connected to the differential input stage, and its drain connected to the output terminal. The drain is also connected through a current source to the reference potential.
A problem with this amplifier circuit is its slow response, due to the limited gain of the differential input stage and the fixed behavior of the current source in the output stage. The slow response is particularly noticeable when the driving voltage must be shifted toward the reference potential.